Generally, to login to a server, you just need to set Username
and Password properties and call Login() method.
All other things will be handled automatically.

Examples:

C#:

pop3.Username = "user";
pop3.Password = "pass";
pop3.Login();

There are several authorization methods available in POP3 protocol: plain (USER/PASS commands), digest (APOP command) and SASL (this one supports a lot of mechanisms).
It's possible to enable or disable any of these authorization methods.
If several authorization methods are allowed, they are used according to their priorities.

If the server supports SASL authorization, it also reports what SASL mechanisms it supports.
Check ExtSASLMechanisms property for the list of the supported mechanisms.
It's possible to enable and disable SASL mechanisms and change their priorities.
The following code snippet shows how to enable OAuth 2.0 authorization mechanism and how to set the highest priority to it.